Centrifugal pumps employ centrifugal force to lift liquids from a lower to a higher level or to produce a pressure. Such pumps typically comprise an impeller consisting of a connecting hub with a number of vanes and shrouds, rotating in a volute collector or casing (See FIGS. 1 and 2). Liquid is drawn into the center of the impeller and is picked up by the vanes and accelerated to a high velocity by rotation of the impeller. The liquid is then discharged by centrifugal force into the casing and out the discharge branch of the casing. When liquid is forced away from the center of the impeller, a vacuum is created, causing more liquid to flow into the center of the impeller. Consequently there is a continuous flow through the pump.
The rotation of the impeller vanes results in a higher pressure in the volute collector than in the suction, which results in flow. This higher pressure has to be sealed against the lower pressure suction on one side and where the shaft (at a lower atmospheric pressure) on the other side enters the collector, to avoid leakage losses and loss of performance. In the case of the shaft, the most common sealing method is to utilize a stuffing box with rings of packing. On the front, or suction side, the most common method of sealing is to utilize a close radial clearance between the impeller and the casing and to employ radial seal rings. For pumps used to pump slurry, however, the sealing problem is more difficult. While radial seal rings are effective in clean water pumping applications, experience with slurry pumps has shown that the particles (solids) being pushed through the gap between the sealing surfaces are thrown off the rotating radial surface of the impeller seal ring, causing high wear to the wetted surfaces of the pump.
Wear occurs mostly as a result of particles impacting or sliding on the wetted surfaces. The amount of wear depends on the particle size, shape, specific gravity of the solids, and sharpness of the solids.
In order to reduce wear, some pumps employ a water flush to dilute and exclude solids, some utilize semi-axial gaps tapering inwardly at an angle, and some utilize clearing vanes protruding out of the front shroud of the impeller into the gap between the impeller and the suction liner, or combination of the above. Each of these, however, has either not satisfactorily solved the problem of wear, or has reduced wear at the expense of pump efficiency.
What is needed, then, is a pump seal that is simple, effective in reducing wear, and that does not impair the performance of the pump.